(1) Field of the Invention:
The present invention relates to a retort-sterilizable laminated pouch. More particularly, the present invention relates to a retort-sterilizable laminated pouch which has an excellent resistance to loading pressure.
(2) Description of the Prior Art:
For the manufacture of retort-sterilizable sealing pouches, there have been used laminated sheets comprising a flexible gas-barrier substrate such as an aluminum foil and a heat-sealable crystalline olefin resin layer formed on one surface of the substrate. Such laminated sheets are piled in the form of a pouch where the olefin resin layers confront each other, the periphery is heat-sealed to form a pouch having an opening on one side, a food or the like content is packed into the pouch from said opening, the opening is sealed after degasification and the packed pouch is sterilized under heating in a sterilizing device called "retort", whereby a package that can be stored at normal temperatures is obtained.
The crystalline olefin resin layer of this laminated sheet is required to act as a heat sealant and also as a material protecting the inner face. Furthermore, since the pouch is subjected to the heat-sterilizing treatment conducted under severe conditions, the crystalline olefin resin layer should satisfy various other requirements.
Low density polyethylene which is used most popularly as the crystalline olefin resin layer is advantageous in that the heat-sealing property is excellent, but it is defective in that both the heat resistance and the resistance to extraction are poor. More specifically, low density polyethylene is defective in that at the heat-sterilizing step conducted at high temperatures, pouch breakage is readily caused and the inner face-protecting property is degraded or the resin component migrates into the packed content to degrade the flavor of the content or the sanitary characteristics. On the other hand, medium density or high density polyethylene is excellent in the heat resistance and the resistance to extraction, but it is defective in that environmental stress cracking is readily caused. This defect leads to a fatal disadvantage that when packed and sealed pouches which have been subjected to sterilization are stored in the state where they are piled up, the heat-sealed portions become brittle and pouches are readily broken under a slight shock.